If you want to look at it from a semantic point of view, dentistry certainly is considered a medical specialty. For example, just today I was browsing the application for a medical student fellowship from the Howard Hughes Medical Institute. Although the fellowship is called a "medical student fellowship," it is also open to all US dental students.
Certainly studying the oral cavity is as much medical related as is the digestive system or any other bodily system.
However, there ARE many differences between dentistry and medicine as it is known and practiced today. Foremost is the fact that dentistry is much more a preventative specialty than any of the medical specialties.
To directly answer your question, however, I would have to say that I don't know. I DO know that the first surgeons were dentists, and that many common techniques in medicine were pioneered and first utilized by dentists. I don't know, however, where medicine and dentistry took different paths.
I DO also know that I'm greatful that dentistry is not a specialty under the umbrella of allopathic or osteopathic medicine. As such dentists do not currently have to endure many of the same tortures that our physician counterparts are experiencing. I *DO* wish dentists were smarter in billing, however, and able to utilize the ICD/CPT treatment coding systems that physicians use, rather than solely the ADA coding systems.
The difference in this lies in the fact that physicians have billing codes for time spent on check-up procedures, whereas dentists by and large only code for procedures that are actually treatment methods.
Now I'm just rambling...